This invention relates to filters, and more particularly to filter elements that are useful in hot gas filter assemblies.
Many processes exist wherein a hot gaseous medium is produced which contains particulate material that must be separated from a gaseous medium, either to prevent pollution, or to remove hazardous material. High temperature filtration of particulates has become an important component in many emerging technologies. Advanced coal conversion technologies, such as fluid bed gasification and combustion, are dependent upon the successful removal of particulates at temperatures in the range of about 500° C. to 1000° C. Other applications which benefit from high temperature filtration range from gas cleaning for biomass gasification to power generation from the incineration of municipal solid wastes. These applications require the removal of particulates from gas streams at high temperature so that process equipment, such as rotating machinery and heat exchangers, which are subjected to the gas streams, remain functional and efficient.
Examples of hot gas filter assemblies are shown in U.S. Pat. Nos. 5,944,859; 5,876,471; 5,453,108; 5,433,771; 4,904,287; and 4,737,176. Hot gas filtration systems used in electric power generating systems protect downstream heat exchangers and gas turbine components from particle fouling and erosion, and clean the process gas to meet emission requirements. When installed in either pressurized fluidized-bed combustion (PFBC), pressurized circulating fluidized-bed combustion (PCFBC), or integrated gasification combined cycle (IGCC) power plants, lower downstream component costs are projected, in addition to improved energy efficiency, lower maintenance, and elimination of additional and expensive fuel or flue gas treatment systems.
The principal function of the hot gas filter elements that are used in either coal-fired or industrial applications has typically been the removal of fine particulates. During gasification, fuel-bound nitrogen in coal is principally released as ammonia (NH3) into the fuel gas stream. With subsequent combustion of the fuel gas in a gas turbine, NH3 forms oxides of nitrogen (NOx), which are difficult to remove hazardous pollutants, and precursors to “acid rain”. The concentration of ammonia in fuel gas can vary from 200 to 5,000 ppmv depending on the nitrogen content of the feedstock coal, as well as the configuration of the gasifier and its operation. The ammonia concentration in the fuel gas depends on the time-dependent history of the gas in the gasifier, with longer residence time at high temperature (>1000° C.) favoring removal of ammonia via thermal decomposition (NH3−>½N2+3/2 H2). The concentration of ammonia is not further reduced after leaving the gasifier due to lowered temperature and shorter residence time in downstream process piping.
Nickel and MoS2-based catalysts have been shown to be capable of decomposing ammonia in hot coal-derived gas streams. In the absence of H2S, decomposition of ammonia can be carried out readily at 550-800° C. using nickel-based catalysts. The ammonia decomposition capability and performance of several catalysts containing Ni, Co, Mo, and W (with Al2O3 and TiO2) has been evaluated in simulated coal-derived gas streams.
Alternative approaches for removal of ammonia have also been described, including: (1) decomposition on the surface of Alloy RA339 in the temperature range of 1200-1300° F. and 1500-1600° F.; (2) decomposition on the surface of an aluminosilicate catalyst in the temperature range of 800-1000° F.; and (3) the use of the RA-330 Promoted Decomposition (RAPD) process whereby fuel gas enters an ammonia decomposition reactor that contains a packed bed of RA330 honeycombs at temperatures of 1500-1600° F. Ammonia decomposition to N2 and H2 on the surface of RA-330, has also been proposed.
In addition to the production of unwanted gasses, aerosol tars that are formed during coal gasification are not readily removed from gas streams by conventional means, and thus often end up plugging filters or fouling fuel cells, turbines, or sorbents. Catalytic cracking of tars to molecular species containing <10% carbon atoms would prevent the problem commonly attributed to the tars. Several approaches to catalytically cracking tars have been proposed, including: the use of dolomites and Englehard extruded and finely divided zeolites, and the use of nickel-based catalysts for hot gas cleaning when the flue gas contains a low tar content.
Nickel-based catalysts have been used to promote reforming and upgrading of the process gas stream generated during biomass gasification. In such systems, ceramic filter elements were used to remove particulates from the 500-600° F. biomass gasification stream.
The process gas then entered a calcined bed of dolomite, where ultimately 90-95 wt % of the tars present in the flue gas were removed, primarily through the reaction of coke+H2O−>CO+H2. As a result, H2 content increased as flue gas passed through the catalytic bed; CO increased in the catalytic reactor; CO2 decreased in the flue gas after going through the catalytic bed; steam content decreased after going through the catalytic bed; and methane decreased in the catalytic bed as a result of steam and dry (CO2) reforming reactions.
Numerous patents relate to the use of catalysts for removal of process gas contaminants primarily through the use fuel-based additives, and catalyst-based reactors. For example, the following United States Patents relate to the reduction of NOx compounds: U.S. Pat. Nos. 5,283,055; 3,885,020; 4,080,433; 4,049,583; 4,001,143; 3,900,428; 3,963,827; 3,895,093; 4,140,655; and 5,552,129. The following United States Patents relate to ammonia reduction: U.S. Pat. Nos. 4,389,339; 3,931,051; 4,812,300; and 4,018,712. The following United States Patents relate to partial oxidation of methane: U.S. Pat. Nos. 4,140,655; 3,895,093; and 5,149,516. The following United States Patents relate to steam reforming and/or hydrocarbon treatment: U.S. Pat. Nos. 4,456,703; 4,179,409; 4,055,513; 4,102,777; 3,885,020; 4,080,433; and 4,049,583. The following United States Patent relates to SOx reduction: U.S. Pat. No. 4,589,978.
It would be desirable to have a filter element that not only removes particulates, but also removes various pollutants and undesirable components of the gas.